A photosensitive drum is generally adopted as an image bearing member in an electrophotographic apparatus which is widely utilized as a dry-type photocopier or a laser printer. The surface of the photosensitive drum is charged, e.g., by corona charger, and an original or the information of a computer output are recorded thereon as an electrostatic latent image by means of light source such as laser beam or the like. The electrostatic latent image is developed into a toner image by a developer using a toner or the like, the toner image, in turn, transferred onto a transfer medium such as a paper sheet by a transfer discharger. After the toner image is fixed into a visible image by a fixing device, the transfer medium is discharged to the outside of the apparatus. Then, the photosensitive drum is ready for forming the next image, after any toner remaining thereon is removed by a cleaner.
In such an electrophotographic apparatus, frequent disassembling of the apparatus or interchanging of the components are inevitably required for the supply of developer such as a toner or the like, discard of the waste toner, cleaning of the discharger, maintenance, or check-up. Moreover, when a so-called jam occurs by inadequate feeding of a transfer medium, it becomes necessary to open the apparatus often to remove the jammed transfer medium.
According to the above, the image formation means are constructed by a kit of a unitary structure in recent years, thereby bringing about convenience in maintenance and checkup. The general process kit comprises a photosensitive drum, a developer, a charger, and a cleaner in a casing which is commonly referred to as a drum cartridge.
A photosensitive layer such as that of a zinc oxide or an organic semiconductor are formed on the photosensitive drum, and this photosensitive layer is liable to be deteriorated by being exposed to severe external light, and to be injured or stained by touching with a hand or a tool. The deterioration, stain, or injury of the photosensitive drum results in lowering the quality of the transfer image.
Therefore, U.S. Pat. No. 4,462,677, No. 4,470,689 or the like, for example, disclose an image formation apparatus having a process unit, or a drum cartridge with a protective cover operable to be opened and closed. When the drum cartridge is mounted to the apparatus, the protective cover is opened, conversely, when the drum cartridge is pulled out from the apparatus, the protect cover is closed for protecting the photosensitive drum.
In the U.S. Pat. No. 4,470,677, however, a user opens and closes the protective cover from the outside of the apparatus by a linear slide mechanism, thus causes mechanical difficulties in fabrication or in use.
Meanwhile, in the U.S. Pat. No. 4,462,677, the protective cover is operable to be opened and closed by a link mechanism driven by the elasticity of a spring and a projection formed thereto. However, since the protective cover of this invention is opened in only one direction, the exposed area of the photosensitive drum becomes reduced. Accordingly, it is disadvantageous in that the transfer device should be installed within a narrow area and that it limits the transfer speed.